com.google.gson.typeadapters

Class RuntimeTypeAdapterFactory<T>

java.lang.Object

com.google.gson.typeadapters.RuntimeTypeAdapterFactory<T>

All Implemented Interfaces:

com.google.gson.TypeAdapterFactory

public final class RuntimeTypeAdapterFactory<T>
extends java.lang.Object
implements com.google.gson.TypeAdapterFactory

Adapts values whose runtime type may differ from their declaration type. This is necessary when a field's type is not the same type that GSON should create when deserializing that field. For example, consider these types:

 abstract class Shape {
   int x;
   int y;
 }
 class Circle extends Shape {
   int radius;
 }
 class Rectangle extends Shape {
   int width;
   int height;
 }
 class Diamond extends Shape {
   int width;
   int height;
 }
 class Drawing {
   Shape bottomShape;
   Shape topShape;
 }
 

Without additional type information, the serialized JSON is ambiguous. Is the bottom shape in this drawing a rectangle or a diamond?

 {
   "bottomShape": {
     "width": 10,
     "height": 5,
     "x": 0,
     "y": 0
   },
   "topShape": {
     "radius": 2,
     "x": 4,
     "y": 1
   }
 }
 

This class addresses this problem by adding type information to the serialized JSON and honoring that type information when the JSON is deserialized:

 {
   "bottomShape": {
     "type": "Diamond",
     "width": 10,
     "height": 5,
     "x": 0,
     "y": 0
   },
   "topShape": {
     "type": "Circle",
     "radius": 2,
     "x": 4,
     "y": 1
   }
 }
 

Both the type field name ("type") and the type labels ("Rectangle") are configurable.

Registering Types

Create a RuntimeTypeAdapterFactory by passing the base type and type field name to the of(java.lang.Class<T>, java.lang.String, boolean) factory method. If you don't supply an explicit type field name, "type" will be used.

 RuntimeTypeAdapterFactory<Shape> shapeAdapterFactory
     = RuntimeTypeAdapterFactory.of(Shape.class, "type");
 

Next register all of your subtypes. Every subtype must be explicitly registered. This protects your application from injection attacks. If you don't supply an explicit type label, the type's simple name will be used.

 shapeAdapterFactory.registerSubtype(Rectangle.class, "Rectangle");
 shapeAdapterFactory.registerSubtype(Circle.class, "Circle");
 shapeAdapterFactory.registerSubtype(Diamond.class, "Diamond");
 

Finally, register the type adapter factory in your application's GSON builder:

 Gson gson = new GsonBuilder()
     .registerTypeAdapterFactory(shapeAdapterFactory)
     .create();
 

Like GsonBuilder, this API supports chaining:

 RuntimeTypeAdapterFactory<Shape> shapeAdapterFactory = RuntimeTypeAdapterFactory.of(Shape.class)
     .registerSubtype(Rectangle.class)
     .registerSubtype(Circle.class)
     .registerSubtype(Diamond.class);
 

Serialization and deserialization

In order to serialize and deserialize a polymorphic object, you must specify the base type explicitly.

 Diamond diamond = new Diamond();
 String json = gson.toJson(diamond, Shape.class);
 

And then:

 Shape shape = gson.fromJson(json, Shape.class);
 

Method Summary

Modifier and Type	Method and Description
<R> com.google.gson.TypeAdapter<R>	create(com.google.gson.Gson gson, com.google.gson.reflect.TypeToken<R> type)
static <T> RuntimeTypeAdapterFactory<T>	of(java.lang.Class<T> baseType) Creates a new runtime type adapter for baseType using "type" as the type field name.
static <T> RuntimeTypeAdapterFactory<T>	of(java.lang.Class<T> baseType, java.lang.String typeFieldName) Creates a new runtime type adapter for baseType using typeFieldName as the type field name.
static <T> RuntimeTypeAdapterFactory<T>	of(java.lang.Class<T> baseType, java.lang.String typeFieldName, boolean maintainType) Creates a new runtime type adapter for baseType using typeFieldName as the type field name.
RuntimeTypeAdapterFactory<T>	recognizeSubtypes() Ensures that this factory will handle not just the given baseType, but any subtype of that type.
RuntimeTypeAdapterFactory<T>	registerSubtype(java.lang.Class<? extends T> type) Registers type identified by its simple name.
RuntimeTypeAdapterFactory<T>	registerSubtype(java.lang.Class<? extends T> type, java.lang.String label) Registers type identified by label.

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Method Detail

of

public static <T> RuntimeTypeAdapterFactory<T> of(java.lang.Class<T> baseType,
                                                  java.lang.String typeFieldName,
                                                  boolean maintainType)

Creates a new runtime type adapter for baseType using typeFieldName as the type field name. Type field names are case sensitive.

Parameters:

maintainType - true if the type field should be included in deserialized objects

of

public static <T> RuntimeTypeAdapterFactory<T> of(java.lang.Class<T> baseType,
                                                  java.lang.String typeFieldName)

Creates a new runtime type adapter for baseType using typeFieldName as the type field name. Type field names are case sensitive.

of

public static <T> RuntimeTypeAdapterFactory<T> of(java.lang.Class<T> baseType)

Creates a new runtime type adapter for baseType using "type" as the type field name.

recognizeSubtypes

@CanIgnoreReturnValue
public RuntimeTypeAdapterFactory<T> recognizeSubtypes()

Ensures that this factory will handle not just the given baseType, but any subtype of that type.

registerSubtype

@CanIgnoreReturnValue
public RuntimeTypeAdapterFactory<T> registerSubtype(java.lang.Class<? extends T> type,
                                                                          java.lang.String label)

Registers type identified by label. Labels are case sensitive.

Throws:

java.lang.IllegalArgumentException - if either type or label have already been registered on this type adapter.

registerSubtype

@CanIgnoreReturnValue
public RuntimeTypeAdapterFactory<T> registerSubtype(java.lang.Class<? extends T> type)

Registers type identified by its simple name. Labels are case sensitive.

Throws:

java.lang.IllegalArgumentException - if either type or its simple name have already been registered on this type adapter.

create

public <R> com.google.gson.TypeAdapter<R> create(com.google.gson.Gson gson,
                                                 com.google.gson.reflect.TypeToken<R> type)

Specified by:

create in interface com.google.gson.TypeAdapterFactory